To allow a further reduction in the widths of structures of electronic components, particularly into the sub-micron range, it is desirable to use shorter wavelengths for the light that is used in the microlithography process. It is conceivable to use light with wavelengths shorter than 100 nm, for example in a lithographic process with soft X-rays, which is referred to as EUV lithography.
The field of EUV lithography is one of the most promising lithographic techniques of the future. Currently under discussion are wavelengths for EUV lithography in the range of 5 nm to 30 nm, especially 11 to 14 nm, in particular 7 nm or 13.5 nm, with a numerical aperture of 0.2 to 0.3. The image quality in the EUV lithography process is determined on the one hand by the projection objective and on the other hand by the illumination system. The illumination system desirably provides the most uniform illumination possible for the field plane in which the structure-carrying mask, the so-called reticle, is arranged.
The projection objective projects an image of the field plane into an image plane, the so-called wafer plane, in which a light-sensitive object is arranged. Projection exposure apparatus for EUV lithography are designed with reflective optical elements. The field in the image plane of an EUV projection exposure apparatus typically has the shape of a ring field with a high aspect ratio, with a width of 2 mm and an arc length of 22 to 26 mm. The projection systems are normally operated in the scanning mode. Concerning the subject of EUV projection exposure apparatus, reference is made to the following publications: W. Ulrich, S. Beiersdörfer, H. J. Mann, “Trends in Optical Design of Projection Lenses for UV- and EUV-Lithography”, in Soft X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (editors), Proceedings of SPIE, Vol. 4146 (2000), pp. 13-24; and M. Antoni, W. Singer, J. Schultz, J. Wangler, I. Escudero-Sanz, B. Kruizinga, “Illumination Optics Design for EUV-Lithography”, in Soft X-Ray and EUV Imaging Systems, W. M. Kaiser, R. H. Stulen (editors), Proceedings of SPIE, Vol. 4146 (2000), pp. 25-34. The entire contents of both of these publications are incorporated by reference herein.
In order to project an image of a mask that is arranged in an image plane of the illumination system onto a light-sensitive substrate, for example a wafer, which can be used for the production of semiconductor elements, it is desirable for the shape of the illumination and the illumination intensity in the plane in which the mask is arranged to be kept constant during the exposure process. A change of the illumination can be caused by a change in power or a change in position as a result of, respectively, a degradation of the light source or a change in the position of the light source.
Particularly in illumination systems having two partial systems, i.e. a first partial system including at least the light source and possibly a collector, and a second partial system including at least one optical element with mirror facets, a change in the radiation characteristics of the light source, a change in the position of the source, or also a centering error or adjustment error, i.e. a misalignment of the first and the second partial system, can cause fluctuations of the illumination of the field in a field plane and/or of the exit pupil in an exit pupil plane of an illumination system, or a loss of uniformity in the field plane and/or a telecentricity error in the exit pupil.
The adjustment error described above between a first partial system including a light source unit and a second partial system including optical components serving for example to illuminate a field plane or a pupil plane represents only one possible out-of-adjustment condition in an optical system, specifically an optical system which finds application for example in microlithography. Another possible adjustment error could for example consist of an out-of-adjustment condition of individual optical components, for example individual mirrors in a projection objective.
A projection exposure apparatus which is described in U.S. Pat. No. 6,842,500 includes a device whereby the strength of the illumination, the so-called exposure dose, can be determined. Also disclosed in U.S. Pat. No. 6,842,500 is an illumination system which has a first partial system including a light source and a collector, wherein the first partial system projects an image of the light source into an intermediate image. The illumination system further includes a second partial system which in the light path from the light source to the exit pupil is arranged downstream of the intermediate image of the light source, wherein the second partial system includes optical components serving to illuminate a field in a field plane and an exit pupil in an exit pupil plane. As is further described in general terms in U.S. Pat. No. 6,842,500, as a way of aligning a first and a second partial system, the illumination device includes a detection device which serves to detect deviations of the positions of the optical axes between the first partial system, the so-called source system, and the second partial system. To implement this concept, U.S. Pat. No. 6,842,500 proposes in general terms to place sensors on a facet mirror that is arranged in the second partial system. To provide the capability for aligning the optical axis dependent on the deviation that has been detected, U.S. Pat. No. 6,842,500 describes the concept of designing the collector mirror to be position-adjustable, so that by controlling the collector mirror it is possible to correct a misalignment of the first and second partial systems. As a sensor concept, it is proposed in U.S. Pat. No. 6,842,500 to register the photo current which is induced by the incident light in the operating light path, for example on the surface of a folding mirror in the light path of the illumination system near the plane in which the reticle is arranged. In the arrangement according to U.S. Pat. No. 6,842,500, a change of the photoelectric effect which occurs on an optical element of the illumination device is always detected at the surface of the individual optical elements in the vicinity of the optical element which also reflects or transmits the incident light of the operating radiation.
An illumination system which is disclosed in WO 2004/031854 includes a first partial optical system and a second partial optical system, wherein the first partial optical system includes the light source and the second partial optical system includes at least one field facet mirror. According to a concept disclosed in WO 2004/031854, sensors are placed on an optical element which is arranged in or near the field plane, for example on the field facet mirror, for the purpose of adjusting the source module and the downstream optical components. WO 2004/031854 also discloses a pupil facet mirror with detectors arranged on it which can be configured as quadrant detectors.
Further illumination systems with detectors are presented in EP A 0 421 746 and in EP A 1 901 125, with the latter reference showing the detectors arranged on an aperture stop.